There is a continuing interest in the safe containment of liquids, particularly gasoline and other fuels, because of the vast amount of fuel presently stored throughout the country. It has become increasingly apparent that underground storage tanks can and do leak, thus causing serious short-term and long-term problems. Therefore, many localities have chosen to examine all underground tanks, mostly fuel storage tanks, to determine if the tanks are leaking. It is an expensive process to inspect the tanks and test the earth surrounding the tanks for indications of leakage. Repair or replacement of tanks which are shown to be leaking increases the cost a great deal further.
To aid the detection and prevention of leaks, double wall (dual containment) tanks have been used for both underground and above ground storage. The space between the two walls of the tanks is monitored for leakage. When used underground, monitors can be used to sense both water seepage in and stored liquid seepage out. If a leak is detected, the tank can be drained and abandoned, dug up and removed, or repaired in place. These options are all quite expensive. If the contaminated soil surrounding the leaking tank must be removed, the expense increases dramatically. However, conventional double wall tanks are quite expensive and are not presently able to meet fire code standards for above-ground storage of flammable liquids. Therefore, even when dual containment tanks are used, flammable liquid is most often stored in below-ground tanks.
Creating a concrete encased hollow tank is not without problems. One conventional method of doing so, described in U.S. patent application Ser. No. 936,205 mentioned above, is to form an open-top hollow shell, place the tank within the hollow shell and then pour the top. However, this process creates cold joints which are possible sources of leaks and weak points in the structure. Therefore, a homogenous concrete shell, which would eliminate cold joints, would be preferable. Entombing a hollow tank in fluid concrete is hindered, if not prevented, by the fact that the hollow tank has a tendency to bob up or float within the fluid concrete. Although the tank could be filled with water to give it neutral buoyancy, this solution causes other problems, including difficulties arising from trying to remove all the water from the tank after the concrete has set.